Heat pump system with flash defrosting mode

ABSTRACT

A heat pump system is provided, comprising: a cooling and heating coil having first and second refrigerant ports; a reheat coil having third and fourth refrigerant ports; first and second refrigerant pipes connected to the first and second refrigerant ports, respectively; a first solenoid valve between the third refrigerant port and the second refrigerant pipe; a second solenoid valve between the fourth refrigerant port and the second refrigerant line; an expansion valve between the fourth refrigeration port and the second refrigerant port; a first check valve between the fourth refrigerant port and the expansion valve; a second check valve between the expansion valve and a condensing circuit; a third check valve between the first check valve and the expansion valve; a fan circuit for blowing air across the cooling and heating coil and the reheat coil in order; and a controller for controlling the heat pump system.

TECHNICAL FIELD

The disclosed devices and methods relate generally to a heating,ventilation, and air-conditioning (HVAC) system. More particularly, thedisclosed devices and methods relate to a HVAC system which quicklydefrosts a condenser by directing warm refrigerant from a reheat coil toa cooling and heating coil to speed up the defrosting of a condenser.

BACKGROUND

An HVAC system may have a condensing circuit and a heating and coolingunit to perform heating and cooling of an indoor area. A condenserlocated in the condensing circuit may collect frost on it during aheating operation in cold weather. When this occurs, it is necessary totemporarily suspend the heating operation to defrost the condenser.

In general, to defrost an outdoor condenser, an HVAC system will switchfrom a heating mode to a defrosting mode. In the defrosting mode, theHVAC system operates in similar fashion to a cooling mode, but fans donot blow air over the condenser or the cooling and heating coil. In aconventional defrosting mode, the heat to melt the frost on the outdoorcondenser comes from the heat of compression produced by the compressionof the refrigerant in the compressor of the heat pump, the heat in therefrigerant piping that has been warmed from operating in heating mode,and the heat from the cooling and heating coil that was warm fromoperating in the heating mode.

As a result, it is necessary to provide an HVAC system with a largercapacity than would otherwise be required to account for this temporarypause in the heating operation during the defrosting time and ensurethat he system can maintain a desired level of heating. It is thereforedesirable to minimize the time needed to defrost the condenser so thatan HVAC system with a smaller capacity may be selected.

SUMMARY OF THE INVENTION

According to one or more embodiments, a heat pump system is provided,comprising: a cooling and heating coil having a first refrigerant portand a second refrigerant port and configured to circulate refrigerant; areheat coil having a third refrigerant port and a fourth refrigerantport and configured to circulate the refrigerant; a plurality ofrefrigerant pipes configured to circulate the refrigerant, the pluralityof refrigerant pipes including a first refrigerant pipe connectedbetween the first refrigerant port and a condensing circuit, a secondrefrigerant pipe connected between the second refrigerant port and thecondensing circuit, a third refrigerant pipe connected between the thirdrefrigerant port and a first node on the second refrigerant pipe, afourth refrigerant pipe connected between the fourth refrigerant portand a second node on the third refrigerant pipe, and a fifth refrigerantpipe connected between a third node on the fourth refrigerant pipe and afourth node on the second refrigerant pipe; a first solenoid valveformed on the third refrigerant pipe between the third refrigerant portand the second node; a second solenoid valve formed on the fourthrefrigerant pipe between the second node and the third node; anexpansion valve connected between the second refrigerant port and thefourth node; a first check valve connected between the fourthrefrigerant port and the third node and configured to prevent flow ofthe refrigerant from the third node to the fourth refrigerant port; asecond check valve connected between the first node and the fourth nodeand configured to prevent flow of the refrigerant from the first node tothe fourth node; a third check valve connected between the third nodeand the fourth node and configured to prevent flow of the refrigerantfrom the fourth node to the third node; a fan circuit configured to blowinput air across the cooling and heating coil to generate discharge airand to blow the discharge air over the reheat coil to generate supplyair; and a controller configured to control the heat pump system.

The expansion valve may be an electronically controlled expansion valve.

The first solenoid valve may be a positive off solenoid valve.

The second solenoid valve may be a positive off solenoid valve.

According to one or more embodiments, a method for operating a heat pumpsystem to defrost a condenser is provided, the method comprising:maintaining refrigerant in a reheat coil without circulating therefrigerant through the reheat coil during a heating mode; circulatingrefrigerant through a cooling and heating coil during the heating mode;blowing input air across the cooling and heating coil during the heatingmode to generate discharge air, the discharge air in the heating modebeing warmer than the input air; blowing the discharge air over thereheat coil during the heating mode to generate supply air; circulatingrefrigerant from the reheat coil to the cooling and heating coil afterentering a defrost mode; and circulating refrigerant from the coolingand heating coil to the condenser coil during the defrost mode.

The method may further comprise stopping blowing the input air acrossthe cooling and heating coil and stopping blowing the discharge air overthe reheat coil after entering a defrost mode.

The method may further comprise stopping blowing the input air acrossthe cooling and heating coil and stopping blowing the discharge air overthe reheat coil in response to entering the defrost mode, wherein thecirculating of the refrigerant from the reheat coil to the cooling andheating coil is performed after the stopping of blowing the input airacross the cooling and heating coil and the stopping of blowing thedischarge air over the reheat coil, and the circulating of therefrigerant from the cooling and heating coil to the condenser coil isperformed after the stopping of blowing the input air across the heatingand cooling coil and the stopping of blowing the discharge air over thereheat coil.

The circulating of the refrigerant from the reheat coil to the coolingand heating coil may be achieved by opening a first solenoid valve andclosing a second solenoid valve.

The method may further comprise receiving from a controller a signalindicating a start of a defrosting mode prior to entering the defrostmode.

The method may further comprise receiving a signal from a controllerindicating an end of a defrosting mode and a resumption of the heatingmode; and stopping circulating refrigerant from the reheat coil to thecooling and heating coil after the defrosting mode has ended and theheating mode has resumed.

The circulating of the refrigerant from the reheat coil to the coolingand heating coil may be achieved by opening a first solenoid valve andclosing a second solenoid valve; and the stopping of the circulating ofthe refrigerant from the reheat coil to the cooling and heating coil maybe achieved by closing the first solenoid valve and opening the secondsolenoid valve.

The method may further comprise resuming blowing input air across thecooling and heating coil to generate discharge air after the defrostingmode has ended and the heating mode has resumed; and resuming blowingthe discharge air over the reheat coil to generate supply air after thedefrosting mode has ended and the heating mode has resumed.

According to one or more embodiments, a non-transitory computer-readablemedium comprising instructions for execution by a computer, theinstructions including a computer-implemented method for controlling aheat pump system to defrost a condenser coil, the instructions forimplementing: maintaining refrigerant in a reheat coil withoutcirculating the refrigerant through the reheat coil during a heatingmode; circulating refrigerant through a cooling and heating coil duringthe heating mode; blowing input air across the cooling and heating coilduring the heating mode to generate discharge air, the discharge air inthe heating mode being warmer than the input air; blowing the dischargeair over the reheat coil during the heating mode to generate supply air;circulating refrigerant from the reheat coil to the cooling and heatingcoil after entering a defrost mode; and circulating refrigerant from thecooling and heating coil to the condenser coil during the defrost mode.

The instructions may be for further implementing stopping blowing theinput air across the cooling and heating coil and stopping blowing thedischarge air over the reheat coil after entering a defrost mode.

The instructions may be for further implementing stopping blowing theinput air across the cooling and heating coil and stopping blowing thedischarge air over the reheat coil in response to entering the defrostmode, wherein the circulating of the refrigerant from the reheat coil tothe cooling and heating coil is performed after the stopping of blowingthe input air across the cooling and heating coil and the stopping ofblowing the discharge air over the reheat coil, and the circulating ofthe refrigerant from the cooling and heating coil to the condenser coilis performed after the stopping of blowing the input air across thecooling and heating coil and the stopping of blowing the discharge airover the reheat coil.

The circulating of the refrigerant from the reheat coil to the coolingand heating coil may be achieved by opening a first solenoid valve andclosing a second solenoid valve.

The instructions may be for further implementing exiting the defrostingmode and resuming the heating mode; and stopping circulating refrigerantfrom the reheat coil to the cooling and heating coil after thedefrosting mode has ended and the heating mode has resumed.

The circulating of the refrigerant from the reheat coil to the coolingand heating coil may be achieved by opening a first solenoid valve andclosing a second solenoid valve; and the stopping of the circulating ofthe refrigerant from the reheat coil to the cooling and heating coil maybe achieved by closing the first solenoid valve and opening the secondsolenoid valve.

The instructions may be for further implementing resuming blowing inputair across the cooling and heating coil to generate discharge air afterthe defrosting mode has ended and the heating mode has resumed; resumingblowing the discharge air over the reheat coil to generate supply airafter the defrosting mode has ended and the heating mode has resumed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements and which together with thedetailed description below are incorporated in and form part of thespecification, serve to further illustrate an exemplary embodiment andto explain various principles and advantages in accordance with thepresent disclosure.

FIG. 1 is a diagram of a heat pump system according to disclosedembodiments;

FIG. 2 is a diagram of a heat pump system operating in a cooling modeaccording to disclosed embodiments;

FIG. 3 is a diagram of a heat pump system operating in a cooling withreheat mode according to disclosed embodiments;

FIG. 4 is a diagram of a heat pump system operating in a heating modeaccording to disclosed embodiments;

FIG. 5 is a diagram of a heat pump system operating in a flash defrostmode according to disclosed embodiments; and

FIG. 6 is a flow chart showing the operation of a heat pump systemoperating in a defrost mode according to disclosed embodiments.

DETAILED DESCRIPTION Heat Pump System

FIG. 1 is a diagram of a heat pump system 100 according to disclosedembodiments. As shown in FIG. 1 , the heat pump system 100 includes acondensing circuit 103, a heating and cooling unit 106, first and secondrefrigerant pipes 180, 182 that connect the condensing circuit 103 andthe heating and cooling unit 106. The condensing circuit 103 includes acompressor 110, a suction accumulator 113, a reversing valve 116, acondenser 118, a condenser fan 119, a first expansion valve 120, asecond expansion valve 123, and a liquid heat exchanger 126. The heatingand cooling unit 106 includes a cooling and heating coil 140 having afirst refrigerant port 143 and a second refrigerant port 146, a reheatcoil 150 having a third refrigerant port 153 and a fourth refrigerantport 156, a cooling and heating coil fan 160, a first solenoid valve164, a second solenoid valve 166, a third expansion valve 168, a firstcheck valve 170, a second check valve 173, a third check valve 176, athird refrigerant pipe 184, a fourth refrigerant pipe 186, a fifthrefrigerant pipe 188, a first node 190, a second node 192, a third node194, a fourth node 196, and a controller 135. The controller 135includes a processor 137 and a memory 139.

The compressor 110 is configured to circulate refrigerant through theheat pump system 100. The compressor 110 receives refrigerant at a lowpressure from the suction accumulator 113, compresses the refrigerant toa higher pressure, thereby heating the refrigerant, and provides therelatively hot, high-pressure refrigerant to the reversing valve 116.

The suction accumulator 113 is disposed at the inlet of the compressor110. The suction accumulator 113 operates as a refrigerant reservoir toprevent liquid refrigerant from entering the compressor 110.

The reversing valve 116 is configured to allow the system to switch thedirection in which refrigerant flows, thereby permitting the heat pumpsystem 100 to perform both heating and cooling operations. The reversingvalve has at least two states in which the outlet of the compressor 110,the condenser 118, the first refrigerant port 143 of the cooling andheating coil 140, and the suction accumulator 113 are selectivelyconnected. In a first state, the reversing valve 116 connects the outletof the compressor 110 with the condenser 118 and connects the inlet ofthe compressor 110 with the first refrigerant port 143 of the coolingand heating coil 140. In a second state, the reversing valve connectsthe inlet of the compressor 110 and the condenser 118 and connects theoutlet of the compressor 110 to the first refrigerant port 143 of thecooling and heating coil 140. The reversing valve 116 may be a four-wayvalve.

The condenser 118 is a heat exchanger that may be located either indoorsor outdoors. The condenser 118 operates to exchange heat betweenrefrigerant flowing through its pipes and air moving past the pipes. Ina heating mode, refrigerant in the condenser 118 will absorb heat frompassing air. In a cooling mode, heat will be transferred from therefrigerant circulated in the condenser 118 to the passing air. When theheat pump system 100 is in a heating mode, the condenser 118 may becomecold enough that frost forms on the pipes of the condenser 118 throughwhich the refrigerant flows.

The condenser fan 119 operates to blow condenser input air (240 in FIGS.2-4 ) through the condenser 118 and past the refrigerant pipes in thecondenser 118. The condenser fan 119 may be located and operated suchthat it draws air through the condenser 118 or blows air throughcondenser 118.

The first expansion valve 120 may be disposed on the second refrigerantpipe 182 between the condenser 118 and the liquid heat exchanger 126.The first expansion valve 120 may be an electronically controlledexpansion valve. The first expansion valve 120 operates to selectivelylower the pressure of the refrigerant passing through it. This drop inpressure will result in a drop in the temperature of the refrigerant.The first expansion valve 120 can be set to be: (a) controlling flow,reducing the pressure of the refrigerant that flows through it; or (b)entirely open, allowing refrigerant to freely flow through it.

The liquid heat exchanger 126 may be a subcooling heat exchanger thatcools liquid refrigerant flowing through the second refrigerant pipe182.

The second expansion valve 123 may be disposed between the secondrefrigerant pipe 182 and a coil of the liquid heat exchanger 126. Thesecond expansion valve 123 may be an electronically controlled expansionvalve. The second expansion valve 123 operates to selectively lower thepressure of the refrigerant passing through it. This drop in pressurewill result in a drop in the temperature of the refrigerant. The secondexpansion valve 123 can be set to be: (a) controlling flow, reducing thepressure of the refrigerant that flows through it; or (b) entirely open,allowing refrigerant to freely flow through it.

The cooling and heating coil 140 is a heat exchanger that may be locatedeither indoors or outdoors. The cooling and heating coil 140 operates toexchange heat between refrigerant flowing through its pipes and airmoving past the pipes. In a heating mode, heat will be transferred fromthe refrigerant circulated in the cooling and heating coil 140 to thepassing air. In a cooling mode, refrigerant in the cooling and heatingcoil 140 will absorb heat from passing air.

The first refrigerant port 143 may act as a refrigerant inlet or arefrigerant outlet for the cooling and heating coil 140 depending uponthe direction of refrigerant flow in the heat pump system 100. Thesecond refrigerant port 146 may act as a refrigerant inlet or arefrigerant outlet for the cooling and heating coil 140 depending uponthe direction of refrigerant flow in the heat pump system 100.

The cooling and heating coil fan 160 blows cooling and heating input air(210 in FIGS. , 2-4 ) through the cooling and heating coil 140 and pastthe refrigerant pipes in the cooling and heating coil 140. The coolingand heating coil 140 exchanges heat with the input air to generatedischarge air (220 in FIGS. 2-4 ), which is output from the cooling andheating coil 140. The cooling and heating input air 210 may be outdoorair, return air drawn from inside of a building, or a mixture of outdoorair and return air. The cooling and heating coil fan 160 may be locatedand operated such that it draws the cooling and heating input air 210through the cooling and heating coil 140 or blows the cooling andheating input air 210 through cooling and heating coil 140.

The reheat coil 150 is a heat exchanger. The reheat coil 150 operates toexchange heat between refrigerant flowing through its pipes and airmoving past the pipes. Specifically, the discharge air 220 will absorbheat from the refrigerant circulating in the reheat coil 150. The reheatcoil 150 may be located adjacent to the cooling and heating coil 140such that air passing through the cooling and heating coil 140subsequently passes through the reheat coil 150.

The third refrigerant port 153 acts as a refrigerant inlet for thereheat coil 150. The fourth refrigerant port 156 may act as arefrigerant outlet for the reheat coil 150.

The cooling and heating coil fan 160 blows the discharge air 220 fromthe cooling and heating coil 140 through the reheat coil 150 and pastthe refrigerant pipes in the reheat could 150 to generate supply air(230 in FIGS. 2-4 ), which can be supplied to an indoor space that isbeing heated or cooled. The cooling and heating coil fan 160 may belocated and operated so that it draws air through the cooling andheating coil 140 and the reheat coil 150 or such that it blows airthrough the cooling and heating coil 140 and the reheat coil 150.

The third refrigerant pipe 184 is a pipe for circulating refrigerant.The third refrigerant pipe 184 extends from the third refrigerant port153 to the first node 190. The third refrigerant pipe 184 connects tothe fourth refrigerant pipe 186 at the second node 192. The thirdrefrigerant pipe connects to the second refrigerant pipe 182 at thefirst node 190.

The fourth refrigerant pipe 186 is a pipe for circulating refrigerant.The fourth refrigerant pipe 186 extends from the fourth refrigerant port156 to the second node 192. The fourth refrigerant pipe 186 connects tothe fifth refrigerant pipe 188 at the third node 194. The fourthrefrigerant pipe 186 connects to the third refrigerant pipe 184 at thesecond node 192.

The fifth refrigerant pipe 188 is a pipe for circulating refrigerant.The fifth refrigerant pipe 188 extends from the third node 194 to thefourth node 196. The fifth refrigerant pipe 188 connects to the fourthrefrigerant pipe 186 at the third node 194 and connects to the secondrefrigerant pipe 182 at the fourth node 196.

The first node 190 is an intersection of the third refrigerant pipe 184and the second refrigerant pipe 182. The second node 192 is anintersection of the third refrigerant pipe 184 and the fourthrefrigerant pipe 186. The third node 194 is an intersection of thefourth refrigerant pipe 186 and the fifth refrigerant pipe 188. Thefourth node 196 is an intersection of the second refrigerant pipe 182and the fifth refrigerant pipe 188.

The first solenoid valve 164 is disposed on the third refrigerant pipe184 between the second node 192 and the third refrigerant port 153 ofthe reheat coil 150. In a fully closed state, the first solenoid valve164 prevents flow of refrigerant through the reheat coil 150. The firstsolenoid valve 164 is a positive off solenoid valve in the embodiment ofFIG. 1 , though it may be a positive on solenoid valve in alternateembodiments.

In alternate embodiments, an expansion valve or other type of valve maybe used in place of a solenoid valve. Solenoid valves are used in theembodiment of FIG. 1 because solenoid valves can only be placed in afully closed state or a fully open state. Other valves, such aselectronic expansion valves, may not close tightly in a “fully closed”state, leading to undesired bleeding of refrigerant through the valve.Using solenoid valves simplifies the controls and reduces incidences ofrefrigerant leaking through valves because solenoid valves can only befully open or fully closed.

The second solenoid valve 166 is disposed on the fourth refrigerant pipe186 between the second node 192 and the third node 194. The secondsolenoid valve 166 is a positive off solenoid valve in the embodiment ofFIG. 1 , though it may be a positive on solenoid valve in alternateembodiments. In alternative embodiments, an expansion valve or othertype of valve may be used in place of a solenoid valve. Solenoid valvesare used because they simplify the controls and reduce incidences ofrefrigerant leaking through valves because solenoid valves can only befully open or fully closed.

The third expansion valve 168 is disposed on the second refrigerant pipe182 between the fourth node 196 and the second refrigerant port 146. Thethird expansion valve 168 may be an electronically controlled expansionvalve. The third expansion valve 168 operates to selectively reduce thepressure of the refrigerant passing through it. This drop in pressurewill result in a drop in the temperature of the refrigerant. The thirdexpansion valve 168 will be set to be controlling flow, reducing thepressure of the refrigerant that flows through it.

The first check valve 170 is disposed on the fourth refrigerant pipe 186between the fourth refrigerant port 156 and the third node 194. Thefirst check valve 170 allows refrigerant to flow in one direction butprevents refrigerant from flowing in the other direction. In the heatpump system 100 of FIG. 1 , the first check valve 170 is arranged toallow the flow of refrigerant from the fourth refrigerant port 156 tothe third node 194 and to prevent the flow of refrigerant from the thirdnode 194 to the fourth refrigerant port 156 and back into the reheatcoil 150.

The second check valve 173 is disposed on the second refrigerant pipe182 between the first node 190 and the fourth node 196. The second checkvalve 173 allows refrigerant to flow in one direction but preventsrefrigerant from flowing in the other direction. In the heat pump system100 of FIG. 1 , the second check valve 173 is arranged to allow the flowof refrigerant from the fourth node 196 to the first node 190 and toprevent the flow of refrigerant from the first node 190 towards thefourth node 196. This allows for the routing of the flow of refrigerantfrom the condensing circuit 103 through the second solenoid valve 166 orthe first solenoid valve 164 when refrigerant is flowing from thecondensing circuit 103 to the heating and cooling unit 106 via thesecond refrigerant pipe 182 (i.e., during a cooling mode or a defrostmode).

The third check valve 176 is disposed on the fifth refrigerant pipe 188between the third node 194 and the fourth node 196. The third checkvalve 176 allows refrigerant to flow in one direction but preventsrefrigerant from flowing in the other direction. In the heat pump system100 of FIG. 1 , the third check valve 176 is arranged to allow the flowof refrigerant from the third node 194 to the fourth node 196 and toprevent the flow of refrigerant from the fourth node 196 to the thirdnode 194.

The controller 135 operates to control the various components in theheat pump system 100. The controller 135 may be located in either of thecondensing circuit 103 and the heating and cooling unit 106. In analternate embodiment, the controller 135 may be located remotely fromthe heat pump system 100. The controller 135 may comprise one or morecontrollers. The controller 135 may comprise one or more processors, oneor more transmitters, one or more receivers, one or more digital signalprocessors, and one or more memory structures. The controller 135 may beprogrammable via a user interface. Although not shown in the drawings,the controller 135 can also have the necessary interface circuitry andconnections to control the operation of various elements in the heatpump system 100. This can include wired and wireless interfaces andconnections.

The controller 135 may, for example, control the operation of thecompressor 110, select the state of the reversing valve 116, set theexpansion amount of the first, second, and third expansion valves 120,123, and 168, open or close the first and second solenoid valves 164 and166, and control the operation of the cooling and heating coil fan 160and the condenser fan 119.

The processor 137 generates signals to perform the control of thecontroller 135. It can store information in the memory 139 and runinstructions stored in the memory 137. The processor can be amicroprocessor (e.g., a central processing unit), anapplication-specific integrated circuit (ASIC), or any suitable devicefor controlling the operation of all or part of the heat pump system100.

The memory 139 can include a read-only memory (ROM), a random-accessmemory (RAM), an electronically programmable read-only memory (EPROM),an electrically erasable programmable read only memory (EEPROM), flashmemory, or any suitable memory device.

Although not shown, the condensing circuit 103 or the heating andcooling unit 106 may include one or more sensors used to determinewhether frost has formed on the condenser 118. These one or more sensorscould include a temperature sensor on or proximate to the condenser 118configured to measure an ambient temperature or a temperature of thecondenser 118, a pressure sensor to monitor the pressure of therefrigerant exiting the condenser 118, a temperature sensor to monitorthe temperature of the refrigerant exiting the condenser 118, or anyother sensor that could provide information that may be used by thecontroller 135 to estimate when frost has formed on the coils of thecondenser 118.

Cooling Operation

FIG. 2 is a diagram of a heat pump system 100 operating in a coolingmode according to disclosed embodiments. During the cooling mode, thefirst solenoid valve 164 is closed (as indicated by it being shown asblack) and the second solenoid valve 166 and the second expansion valve123 are open (as indicated by them being shown as white). As shown inFIG. 2 , the first and second check valves 170, 173 operate to preventthe flow of refrigerant (as indicated by them being shown as beingpartially black) and the third check valve 176 passes refrigerant (asindicated by it being shown as white).

In a cooling mode of the heat pump system 100, hot refrigerant gasleaves the outlet of the compressor 110 and enters the condenser 118.Condenser input air 240 is blown over the condenser 118 by the condenserfan 119. The refrigerant transfers heat to the condenser input air 240and leaves the condenser 118 as a liquid. The liquid refrigerant issubsequently subcooled in a first coil of the liquid heat exchanger 126.A portion of the subcooled refrigerant is diverted through the secondexpansion valve 123 and into a second coil of the liquid heat exchanger126 to absorb heat from the refrigerant that is flowing through thefirst coil of the liquid heat exchanger 126. The remainder of thesubcooled refrigerant flows to first node 190.

The second check valve 173 prevents refrigerant flow from the first node190 to the fourth node 196, and the first solenoid valve 164, which isset to a fully closed state, prevents refrigerant flow to the thirdrefrigerant port 153 and through the reheat coil 150. The subcooledrefrigerant flows through the second solenoid valve 166, which is set ina fully open state, to the third node 194. The first check valve 170prevents flow to the fourth refrigerant port 156 and through the reheatcoil 150.

The refrigerant flows through the third check valve 176 and the thirdexpansion valve 168 and enters the second refrigerant port 146 of thecooling and heating coil 140. The cooling and heating coil fan 160 blowscooling and heating input air 210 over the cooling and heating coil 140,allowing the refrigerant in the cooling and heating coil 140 to absorbheat from the cooling and heating input air 210. As it absorbs heat fromthe input air 210, the refrigerant boils to become a gas. The gaseousrefrigerant exits the cooling and heating coil 140 at the firstrefrigerant port 143 and flows down the first refrigerant pipe 180 andthrough the reversing valve 116. The refrigerant flows to the suctionaccumulator 113 and gaseous refrigerant from the suction accumulator 113enters the inlet of the compressor 110.

In the cooling mode of the heat pump system 100, the controller 135varies the cooling capacity of the air conditioner 100 by varying thespeed of the compressor 110. Changing the speed of the compressor 110changes the temperature of the cooling and heating coil 140.

Cooling With Reheat Operation

FIG. 3 is a diagram of a heat pump system 100 operating in a coolingwith reheat mode. Cooling with reheat mode is used in situations whenhumidity is high, but the outdoor temperature is not high enough for theheat pump system 100 to provide dehumidification of the air withoutcooling it excessively. In this mode, the air is cooled fordehumidification and is reheated before it enters the air-conditionedspace to ensure the comfort of the occupants. During the cooling mode,the first solenoid valve 164 is open (as indicated by it being shown aswhite) and the second solenoid valve 166 and the second expansion valveare closed (as indicated by them being shown in black). As shown in FIG.3 , the second check valve 173 operates to prevent the flow ofrefrigerant (as indicated by it being shown as being partially black)and the first and third check valves 170, 176 pass refrigerant (asindicated by them being shown as white).

In a cooling with reheat mode of the heat pump system 100, hot gaseousrefrigerant leaves the outlet of the compressor 110 and enters thecondenser 118 via the reversing valve 116. The reversing valve 116 is ina state such that the refrigerant pipe 180 is connected to the suctionaccumulator 113 at the inlet of the compressor 110 and the outlet of thecompressor 110 is connected to the condenser 118. Condenser input air240 is blown over the condenser 118 by the condenser fan 119. Therefrigerant transfers heat to the condenser input air 240 while it is inthe condenser 118 and leaves the condenser 118 as a liquid at a hightemperature. The controller 135 may control the amount of heattransferred from the refrigerant in the condenser 118 by controlling thespeed of the condenser fan 119.

The liquid refrigerant flows through the first expansion valve 120 tothe first node 190. The second check valve 173 prevents flow ofrefrigerant from the first node 190 to the fourth node 196 and thesecond solenoid valve 166 is closed to prevent flow of refrigerant fromthe second node 192 to the third node 194. The hot liquid refrigerantflows through the first solenoid valve 164, which is disposed in an openposition. The refrigerant flows through the third refrigerant pipe 184and enters the reheat coil 150 at the third refrigerant port 153. Whenit enters the reheat coil 150, the refrigerant may be a hot liquid or amixture of liquid and gas. The cooling and heating coil fan 160 isoperated to blow discharge air 220 that has been cooled by the coolingand heating coil 140 over the reheat coil 150. The discharge air 220absorbs heat from the refrigerant that is flowing through the reheatcoil 150 to be reheated to an appropriate temperature and becomes supplyair 230 that is provided to an area being cooled.

The refrigerant exits the reheat coil 150 via the fourth refrigerantport 156 and flows through the first check valve 170 to the third node194. The refrigerant flows through the fifth refrigerant pipe 188 viathe third check valve 176 to the fourth node 196 where it then entersthe second refrigerant pipe 182 and travels through the third expansionvalve 168. The third expansion valve is set to lower the pressure of therefrigerant passing through it to thereby lower the temperature of thecoolant.

The relatively lower-temperature refrigerant then enters the cooling andheating coil 140 through the second refrigerant port 146. The coolingand heating coil fan 160 is operated to blow cooling and heating inputair 210 over the cooling and heating coil 140. The refrigerant that isflowing through the cooling and heating coil 140 absorbs heat from thecooling and heating input air 210. This cools the cooling and heatinginput air 210, which becomes discharge air 220.

The refrigerant exits the cooling and heating coil 140 via the firstrefrigerant port 143 after absorbing heat from the cooling and heatinginput air 210 and flows through the first refrigerant pipe 180. Therefrigerant then enters the suction accumulator 113 via the reversingvalve 116. Gaseous refrigerant from the suction accumulator 113 entersthe inlet of the compressor 110, which continues the process. Thecontroller 135 may control the speed of the compressor 110 to vary theamount of cooling provided by the heat pump system.

In the cooling with reheat operation, the cooling and heating coil 140cools the input air 210 below a desired cooling temperature, resultingin discharge air 220 that is colder than is desired for the supply air230. In some embodiments this may be done to dehumidify the input air210. However, the reheat coil 150 heats the discharge air 220 such thatthe supply air 230 is the desired temperature.

Heating Operation

FIG. 4 is a diagram of a heat pump system 100 operating in a heatingmode according to disclosed embodiments. In a heating mode of the heatpump system 100, hot gas refrigerant is discharged from an outlet of thecompressor 110. The reversing valve 116 is disposed such that the outletof the compressor 110 is connected to the first refrigerant pipe 180 andthe condenser 118 is connected to the inlet of the compressor 110.During the cooling mode, the second expansion valve 123 is controlling(as indicated by it being shown as white) and both the first solenoidvalve 164 and the second solenoid valve 166 are closed (as indicated bythem being shown as black). As shown in FIG. 4 , the first and thirdcheck valves 170, 176 operate to prevent the flow of refrigerant (asindicated by them being shown as being partially black) and the secondcheck valve 173 passes refrigerant (as indicated by it being shown aswhite).

In the heating mode of the heat pump system 100, hot gaseous refrigerantleaves the outlet of the compressor 110 and enters the cooling andheating coil 140 via the reversing valve 116. The reversing valve 116 isin a state such that the refrigerant pipe 180 is connected to the outletof the compressor 110 and the inlet of the suction accumulator 113 isconnected to the condenser 118. The relatively hot refrigerant flowsthrough the first refrigerant pipe 180 to the first refrigerant port 143of the cooling and heating coil 140 and into the cooling and heatingcoil 140. The cooling and heating coil fan 160 blows cooling and heatinginput air 210 over the cooling and heating coil 140. The cooling andheating input air 210 absorbs heat from the refrigerant flowing throughthe cooling and heating coil 140 and becomes discharge air 220. Thecooling and heating coil fan 160 blows the discharge air 220 over thereheat coil 150.

The refrigerant is a subcooled liquid when it exits the cooling andheating coil 140 at the second refrigerant port 146. The subcooledrefrigerant flows through the third expansion valve 168, which iscontrolled by the controller 135 to vary the amount of flow allowedthrough the cooling and heating coil 140. The refrigerant flows throughthe second check valve 173. A first portion of the refrigerant flowsthrough a coil of the liquid heat exchanger 126 and is furthersubcooled. A second portion of refrigerant travels through the secondexpansion valve 123 and through the second coil of the liquid heatexchanger 126, where it absorbs heat from the first portion of therefrigerant. The second portion of the refrigerant then flows to thesuction accumulator 113.

The first portion of the refrigerant flows through the first expansionvalve 120 and into the condenser 118. Condenser input air 240 is blownacross the condenser 118 by the condenser fan 119, and the refrigerantwithin the condenser 118 absorbs heat from the condenser input air 240and becomes a vapor. The refrigerant vapor exits the condenser 118 andflows to the suction accumulator 113. Refrigerant vapor flows from thesuction accumulator 113 to the inlet of the compressor 110. Duringheating mode, refrigerant does not flow through the reheat coil 150because the first solenoid valve 164 is closed.

The reheat coil 150 does not circulate any refrigerant during theheating mode, so it does not perform any ongoing heat exchange with thedischarge air 220. During most of the heating mode, the reheat coilsimple passes the discharge air 220 as supply air 230. However, at thebeginning of the heating mode, the refrigerant in the reheat coil 150may be at a lower temperature than the discharge air 220. If this is thecase, the refrigerant in the reheat coil 150 will absorb heat from thedischarge air 220 until it is at a temperature at which heat exchangebetween the discharge air 220 and the refrigerant in the reheat coil 150is negligible. Since the refrigerant in the reheat coil 150 is notcirculating, no little heat is required to maintain the refrigerant inthe reheat coil 150 at this relatively higher temperature throughout theheating mode.

Defrost Mode

FIG. 5 is a diagram of a heat pump system operating in a defrost modeaccording to disclosed embodiments. During the defrost mode, the firstsolenoid valve 164 is open (as indicated by it being shown as white) andthe second solenoid valve 166 and the second expansion valve 123 areclosed (as indicated by them being shown as black). As shown in FIG. 4 ,the second check valve 173 operate to prevent the flow of refrigerant(as indicated by it being shown as being partially black) and the firstand third check valves 170, 176 pass refrigerant (as indicated by thembeing shown as white).

In a defrost mode of the heat pump system 100, the reversing valve 116is disposed such that the outlet of the compressor 110 is connected tothe condenser 118 and the inlet of the suction accumulator 113 isconnected to the first refrigerant pipe 180. Hot gas refrigerant exitsthe compressor 110 and enters the condenser 118 via the reversing valve116.

During the defrost mode, the condenser fan 119 is set to stop the flowof air across the condenser 118 (represented by the stopped condenserinput air 540 in FIG. 5 ). As a result there is no condenser input air240 for the refrigerant to exchange heat with as it passes over thecondenser 118, However, there will generally be frost formed on thecondenser 118, which will be colder than the refrigerant flowing throughcondenser 118. Any frost that has accumulated on the condenser 118therefore absorbs heat from the refrigerant and melts off the condenser118. Having given up heat to melt the frost off the condenser 118, therefrigerant in the condenser 118 condenses to become a liquid.

The refrigerant then flows from the condenser 118 through the firstexpansion valve 120 and the liquid heat exchanger 126, where the liquidrefrigerant may be cooled further. The liquid refrigerant flows from theliquid heat exchanger 126, through the first node 190 and through thefirst solenoid valve 164, which is in a fully open state, to the thirdrefrigerant port 153. The second solenoid valve 166, which is in a fullyclosed state, prevents the refrigerant from flowing from the first node190 to the third node 194 and the second check valve 173 preventsrefrigerant flow from flowing from the first node 190 to the fourth node196. In some embodiments the temperature of the refrigerant flowingbetween the condenser 118 and the reheat coil 150 during the defrostmode may be 10° F. to 60° F. However, this is just by way of example.The refrigerant flowing between the condenser 118 and the reheat coil150 during the defrost mode may be different temperatures in somealternate embodiments.

Since the defrost mode is always entered into during a temporary breakin the heating mode, the reheat coil 150 will typically be filled withwarm liquid refrigerant that was heated by discharge air 220 blowingover the reheat coil 150 during the heating mode. The warm liquidrefrigerant flows out of the reheat coil 150 via the fourth refrigerantport 156. The temperature of the refrigerant exiting the reheat coil 150will be relatively high compared to the refrigerant being provided alongthe second refrigerant line from the condensing circuit 103 (e.g., 70°F. to 105° F.).

This relatively high-temperature refrigerant flows out of the reheatcoil 150 via the fourth refrigerant port 156, through the first checkvalve 170, the third check valve 176, and to the third expansion valve168. As it passes through the third expansion valve 168, the refrigerantflashes, or begins to boil, due to the drop in pressure. Because therefrigerant has been warmed while inside the reheat coil 150, a largerproportion of the refrigerant boils than it otherwise would if coldrefrigerant was used. The refrigerant that is a mixture of gas andliquid enters the cooling and heating coil 140 through the secondrefrigerant port 146. The cooling and heating coil 140 is warm fromoperating in the heating mode. The refrigerant absorbs heat from thewarm cooling and heating coil 140 and the remainder of the liquidrefrigerant boils to become a vapor. The refrigerant evaporatingtemperature inside the cooling and heating coil 140 during thedefrosting mode may be -10° F. to 40° F. Feeding the cooling and heatingcoil 140 with refrigerant that is already warm and that has alreadybecome partially gaseous adds to the total heat available in the coolingand heating coil 140 to defrost the condenser 118 and greatly shortensthe necessary defrost time.

The refrigerant vapor exits the cooling and heating coil 140 through thefirst refrigerant port 143. The refrigerant vapor flows through thefirst refrigerant pipe 180 to the reversing valve 116 and the suctionaccumulator 113 and enters the compressor 110.

During the defrost mode, the cooling and heating fan coil 160 is set tostop the flow of air across the cooling and heating coil 140(represented by the stopped heating and cooling input air 510, stoppeddischarge air 520, and stopped supply air 530 in FIG. 5 ). As a result,there is no heating and cooling input air 240 for the refrigerant toexchange heat with as it passes over the cooling and heating coil 140.

Method of Operation of a Heat Pump System in a Defrost Mode

FIG. 6 is a flow chart 600 showing the method of operation of a heatpump system in a defrost mode according to disclosed embodiments. Themethod begins with the heat pump system 100 entering a heating mode instep 605. In a heating mode of the heat pump system 100, refrigerant ismaintained in a reheat coil 150, but refrigerant is not circulatedthrough the reheat coil 150. (Step 610) This can be achieved in variousembodiments by having valves (e.g., solenoid valves 164, 166) at therefrigerant ports 153, 156 of the reheat coil 150 and setting thosevalves to be fully closed during the heating mode, which can prevent thecirculation of refrigerant through the reheat coil 150.

Refrigerant is then circulated through the cooling and heating coil 140during the heating mode.(Step 615) This refrigerant will generally bewarmer than the cooling and heating air 210 that will be passed over theheating and cooling coil 140 during the heating mode.

Cooling and heating input air 210 is blown across the cooling andheating coil 140 during the heating mode to generate discharge air 220.(Step 620) In an exemplary embodiment, this is accomplished by operatingthe cooling and heating coil fan 160. During this operation heat isexchanged between the refrigerant in the cooling and heating coil 140and the cooling and heating input air 210 such that the discharge air220 is warmer than the cooling and heating input air 210.

The discharge air 220 is blown over the reheat coil 150 during a heatingmode to generate supply air 230. (Step 625) In an exemplary embodiment,this is accomplished by operating the cooling and heating coil fan 160.Since the refrigerant in the reheat coil 150 is not circulated, thetemperature of the supply air 230 will be essentially the same as thetemperature of the discharge air 220, except at the very beginning ofthe heating mode when the refrigerant in the reheat coil 150 may need tobe initially heated to a temperature at which it will no longer exchangeany significant heat with the discharge air 220 passing through thereheat coil 150.

The heat pump system 100 then moves from a heating mode to a defrostmode. (Step 630) In an exemplary embodiment, this may occur when theheat pump system 100 receives a signal from the controller 135indicating a start of a defrost mode. This may be in response to one ormore sensors signals indicative of frost having formed on coils of acondenser, a timer, or any other triggering signal.

Upon entering the defrost mode, the cooling and heating input air 210stops blowing across the cooling and heating coil 140. In an exemplaryembodiment, this is accomplished by stopping the operation of thecooling and heating coil fan 160. (Step 635)

Likewise, upon entering the defrost mode, the discharge air 220 stopsblowing over the reheat coil 150. In an exemplary embodiment, this isaccomplished by stopping the operation of the cooling and heating coilfan 160. (Step 640)

In many embodiments a single cooling and heating fan 160 will be used toboth blow the cooling and heating input air 210 across the cooling andheating coil 140 and to blow the discharge air 220 over the reheat coil150. In this case, steps 635 and 640 can be performed simultaneously bystopping operation of the cooling and heating fan 160.

Refrigerant from the reheat coil 150 is circulated to the cooling andheating coil 140 during the defrost mode. (645) In some embodiments, thetemperature of the refrigerant exiting the reheat coil 150 may be 70° F.to 105° F. (though this can vary in alternate embodiments based on thetemperature of the discharge air 220 in the heating mode). In contrast,the evaporating temperature in the cooling and heating coil 140 may be-10° F. to 40° F.

Circulating the refrigerant from the reheat coil 150 to the cooling andheating coil 140 may be achieved by operating valves that were used toisolate the reheat coil 150 during the heating mode (e.g., by openingthe first solenoid valve 164 and closing the second solenoid valve 166in the embodiment of FIGS. 1-5 ).

Refrigerant from the cooling and heating coil 140 is also circulated tothe condenser 118 in the defrost mode. (Step 650) The refrigerant fromthe cooling and heating coil 140 is initially warm in the defrost modebecause the heat pump system 100 was operating in heating modeimmediately prior to entering the defrost mode. The warm refrigerantcirculated to the condenser 118 warms the condenser 118 and causes frostthat has formed on the condenser 118 to begin melting.

The refrigerant that was maintained in the reheat coil 150 without beingcirculated during the heating mode was also heated by the warm dischargeair 220 blown over the reheat coil 150 by the cooling and heating coilfan 160 during the heating mode. As this relatively warm refrigerant isprovided to the cooling and heating coil 140 during the defrost mode, itwill increase the amount of heat available to the cooling and heatingcoil 140 relative to what would have been available if the cooling andheating coil 140 received only refrigerant from the condensing circuit103. This will allow the cooling and heating coil 140 to provide heat tothe condenser 118 more quickly and can reduce the length of a defrostmode necessary to remove frost from the coils of the condenser 118.

For example, the refrigerant that was warmed in the reheat coil 150could pass through an expansion valve (e.g., third expansion valve 168in FIGS. 1-5 ). As it passes through the expansion valve, therefrigerant flashes, or begins boiling, to become a mixture of gas andliquid. The refrigerant enters the cooling and heating coil 140, whichis warm from operating in a heating mode, and the remainder of theliquid refrigerant absorbs heat from the cooling and heating coil 140and boils off. Because the refrigerant was already warm when it beganboiling, less heat is absorbed by the refrigerant from the cooling andheating coil 140 than would otherwise be absorbed by refrigerant thatwas cold prior to passing though the third expansion valve 168. Thismaximizes the amount of heat available in the cooling and heating coil140 to be used to defrost the condenser 118 and reduces the defrostingtime.

Although the operation of circulating refrigerant from the cooling andheating coil 140 to the condenser 118 (step 645) is listed before theoperation of circulating refrigerant from the reheat coil 150 to thecooling and heating coil 140 (Step 650), these processes are typicallyperformed at the same time and continually.

After a time, the heat pump system 100 moves from the defrost mode backto the heating mode. (Step 655) This may occur after a set period oftime or in response to sensor signals similar to those that triggeredthe defrost mode to begin with. In an exemplary embodiment, the heatpump system 100 may receive a signal from a controller 135 indicating anend of the defrosting mode and a resumption of the heating mode. Once inthe heating mode the system will again stop circulating refrigerant fromthe reheat coil 150 to the cooling and heating coil 140 (Step 610),resuming circulating refrigerant through the heating and cooling coil140 (Step 615), resuming blowing cooling and heating input air 210across the cooling and heating coil 140 to generate discharge air 220(Step 620), and resuming blowing discharge air 220 over the reheat coil150 to generate supply air 230 (625).

The various embodiments which demonstrate a method for controlling aheat pump system have been discussed in detail above. It should befurther noted that the above-described processes can be stored asinstructions in computer-readable storage medium. When the instructionsare executed by a computer (e.g., a processor 137 in a controller 135),for example after being loaded from a computer-readable storage medium(e.g., a memory 139 in a controller 135), the process(es) are performed.In one or more embodiments, a non-transitory computer readable mediummay be provided which comprises instructions for execution by acomputer, the instructions including a computer-implemented method forcontrolling an air-conditioning system to defrost a condenser coil, asdescribed above. The non-transitory computer readable medium maycomprise, for example, a read-only memory (ROM), a random-access memory(RAM), a programmable ROM (PROM), and/or an electrically erasableread-only memory (EEPROM).

Conclusion

This disclosure is intended to explain how to fashion and use variousembodiments in accordance with the invention rather than to limit thetrue, intended, and fair scope and spirit thereof. The foregoingdescription is not intended to be exhaustive or to limit the inventionto the preci se form disclosed. Modifications or variations are possiblein light of the above teachings. The embodiment(s) was chosen anddescribed to provide the best illustration of the principles of theinvention and its practical application, and to enable one of ordinaryskill in the art to utilize the invention in various embodiments andwith various modifications as are suited to the particular usecontemplated. All such modifications and variations are within the scopeof the invention as determined by the appended claims, as may be amendedduring the pendency of this application for patent, and all equivalentsthereof, when interpreted in accordance with the breadth to which theyare fairly, legally, and equitably entitled. The various circuitsdescribed above can be implemented in discrete circuits or integratedcircuits, as desired by implementation.

1. A heat pump system comprising: a cooling and heating coil having afirst refrigerant port and a second refrigerant port and configured tocirculate refrigerant; a reheat coil having a third refrigerant port anda fourth refrigerant port and configured to circulate the refrigerant; aplurality of refrigerant pipes configured to circulate the refrigerant,the plurality of refrigerant pipes including a first refrigerant pipeconnected between the first refrigerant port and a condensing circuit, asecond refrigerant pipe connected between the second refrigerant portand the condensing circuit, a third refrigerant pipe connected betweenthe third refrigerant port and a first node on the second refrigerantpipe, a fourth refrigerant pipe connected between the fourth refrigerantport and a second node on the third refrigerant pipe, and a fifthrefrigerant pipe connected between a third node on the fourthrefrigerant pipe and a fourth node on the second refrigerant pipe; afirst solenoid valve formed on the third refrigerant pipe between thethird refrigerant port and the second node; a second solenoid valveformed on the fourth refrigerant pipe between the second node and thethird node; an expansion valve connected between the second refrigerantport and the fourth node; a first check valve connected between thefourth refrigerant port and the third node and configured to preventflow of the refrigerant from the third node to the fourth refrigerantport; a second check valve connected between the first node and thefourth node and configured to prevent flow of the refrigerant from thefirst node to the fourth node; a third check valve connected between thethird node and the fourth node and configured to prevent flow of therefrigerant from the fourth node to the third node; a fan circuitconfigured to blow input air across the cooling and heating coil togenerate discharge air and to blow the discharge air over the reheatcoil to generate supply air; and a controller configured to control theheat pump system.
 2. The heat pump system of claim 1, wherein theexpansion valve is an electronically controlled expansion valve.
 3. Theheat pump system of claim 1, wherein the first solenoid valve is apositive off solenoid valve.
 4. The heat pump system of claim 1, whereinthe second solenoid valve is a positive off solenoid valve.
 5. A methodfor operating a heat pump system to defrost a condenser coil, the methodcomprising: maintaining refrigerant in a reheat coil without circulatingthe refrigerant through the reheat coil during a heating mode;circulating refrigerant through a cooling and heating coil during theheating mode; blowing input air across the cooling and heating coilduring the heating mode to generate discharge air, the discharge air inthe heating mode being warmer than the input air; blowing the dischargeair over the reheat coil during the heating mode to generate supply air;circulating refrigerant from the reheat coil to the cooling and heatingcoil after entering a defrost mode; and circulating refrigerant from thecooling and heating coil to the condenser coil during the defrost mode.6. The method of claim 5, further comprising: stopping blowing the inputair across the cooling and heating coil and stopping blowing thedischarge air over the reheat coil after entering a defrost mode.
 7. Themethod of claim 5, further comprising stopping blowing the input airacross the cooling and heating coil and stopping blowing the dischargeair over the reheat coil in response to entering the defrost mode,wherein the circulating of the refrigerant from the reheat coil to thecooling and heating coil is performed after the stopping of blowing theinput air across the cooling and heating coil and the stopping ofblowing the discharge air over the reheat coil, and the circulating ofthe refrigerant from the cooling and heating coil to the condenser coilis performed after the stopping of blowing the input air across theheating and cooling coil and the stopping of blowing the discharge airover the reheat coil.
 8. The method of claim 5, wherein the circulatingof the refrigerant from the reheat coil to the cooling and heating coilis achieved by opening a first solenoid valve and closing a secondsolenoid valve.
 9. The method of claim 5, further comprising receivingfrom a controller a signal indicating a start of a defrosting mode priorto entering the defrost mode.
 10. The method of claim 5, furthercomprising: receiving a signal from a controller indicating an end of adefrosting mode and a resumption of the heating mode; and stoppingcirculating refrigerant from the reheat coil to the cooling and heatingcoil after the defrosting mode has ended and the heating mode hasresumed.
 11. The method of claim 10, wherein the circulating of therefrigerant from the reheat coil to the cooling and heating coil isachieved by opening a first solenoid valve and closing a second solenoidvalve; and the stopping of the circulating of the refrigerant from thereheat coil to the cooling and heating coil is achieved by closing thefirst solenoid valve and opening the second solenoid valve.
 12. Themethod of claim 10, further comprising: resuming blowing input airacross the cooling and heating coil to generate discharge air after thedefrosting mode has ended and the heating mode has resumed; resumingblowing the discharge air over the reheat coil to generate supply airafter the defrosting mode has ended and the heating mode has resumed.13. A non-transitory computer-readable medium comprising instructionsfor execution by a computer, the instructions including acomputer-implemented method for controlling a heat pump system todefrost a condenser coil, the instructions for implementing: maintainingrefrigerant in a reheat coil without circulating the refrigerant throughthe reheat coil during a heating mode; circulating refrigerant through acooling and heating coil during the heating mode; blowing input airacross the cooling and heating coil during the heating mode to generatedischarge air, the discharge air in the heating mode being warmer thanthe input air; blowing the discharge air over the reheat coil during theheating mode to generate supply air; circulating refrigerant from thereheat coil to the cooling and heating coil after entering a defrostmode; and circulating refrigerant from the cooling and heating coil tothe condenser coil during the defrost mode.
 14. The non-transitorycomputer-readable medium, as recited in claim 13, the instructions forfurther implementing: stopping blowing the input air across the coolingand heating coil and stopping blowing the discharge air over the reheatcoil after entering a defrost mode.
 15. The non-transitorycomputer-readable medium, as recited in claim 13, the instructions forfurther implementing: stopping blowing the input air across the coolingand heating coil and stopping blowing the discharge air over the reheatcoil in response to entering the defrost mode, wherein the circulatingof the refrigerant from the reheat coil to the cooling and heating coilis performed after the stopping of blowing the input air across thecooling and heating coil and the stopping of blowing the discharge airover the reheat coil, and the circulating of the refrigerant from thecooling and heating coil to the condenser coil is performed after thestopping of blowing the input air across the cooling and heating coiland the stopping of blowing the discharge air over the reheat coil. 16.The non-transitory computer-readable medium, as recited in claim 13,wherein the circulating of the refrigerant from the reheat coil to thecooling and heating coil is achieved by opening a first solenoid valveand closing a second solenoid valve.
 17. The non-transitorycomputer-readable medium, as recited in claim 13, the instructions forfurther implementing: exiting the defrosting mode and resuming theheating mode; and stopping circulating refrigerant from the reheat coilto the cooling and heating coil after the defrosting mode has ended andthe heating mode has resumed.
 18. The non-transitory computer-readablemedium, as recited in claim 17, wherein the circulating of therefrigerant from the reheat coil to the cooling and heating coil isachieved by opening a first solenoid valve and closing a second solenoidvalve; and the stopping of the circulating of the refrigerant from thereheat coil to the cooling and heating coil is achieved by closing thefirst solenoid valve and opening the second solenoid valve.
 19. Thenon-transitory computer-readable medium, as recited in claim 17, theinstructions for further implementing: resuming blowing input air acrossthe cooling and heating coil to generate discharge air after thedefrosting mode has ended and the heating mode has resumed; and resumingblowing the discharge air over the reheat coil to generate supply airafter the defrosting mode has ended and the heating mode has resumed.